White light emitting diodes (hereinafter “LED” or “LEDs”) in lighting constitutes a technological revolution. LED technology provides several benefits compared to lighting technology traditionally used. One major benefit of LED is its great life span that limits maintenance fees associated with its use. Another benefit of LED is that it is easily controlled or adjusted. LEDs offer great luminous efficiency.
Maximal luminous efficiency is currently achieved by LEDs emitting in the visible blue spectrum. In order to yield white lighting using LEDs, the most efficient approach consists of exciting a phosphorous coat using a blue LED. Phosphorus allows the conversion of an important part of blue light into white light (broad spectrum light). Therefore, electromagnetic spectrum of a white LED can be described as a superposition of a white spectrum covering a large part of the visible with a blue peak (see dotted curve in FIG. 5).
Presence of a blue peak in the resulting spectrum poses important problems with respect to the environment, road safety and human health. Indeed, scattering of blue light is more efficient than yellow light or red light. Blue light will therefore tend to cause more astronomical light pollution, especially for an observer located nearby the light source, because of its great scattering efficiency in the atmosphere than any other color of longer wavelength. A second positive impact of reducing the quantity of blue is linked to glare when light is scattering into the human eye. This phenomenon is growing with the ageing of the population, because with age, eyes become less transparent. Glare is a potential cause of road accidents. Less blue light is also less attractive for insects. This filter should therefore allow for a considerable reduction of insect mortality around street lights. It has been estimated that mortality rates are around 150 insects a night per street light.
Moreover, blue light plays an important role in circadian cycle regulation (biological clock) in several species, including humans. Indeed, regulation of this cycle in humans is achieved in part by a photoreceptor located in the eye retina that does not contribute to vision. When this photoreceptor is stimulated, melatonin production is suppressed and therefore conditioning the wake cycle. On the opposite, in the absence of blue light, melatonin is secreted, placing the organism in a state of rest. In addition to this regulation effect on the circadian cycle, melatonin is also a powerful antioxidant that allows the reparation of pre-cancer cells (as demonstrated in mice), therefore reducing risks of developing some types of cancers.
Brainard et al. 2001 and Thapan et al. 2001 characterized spectral response of melatonin suppression. This function is called Melatonin Suppression Action Spectrum (hereinafter “MSAS”). MSAS-based light filtering reduces negative impacts on human health from melatonin suppression cause by exposure to LEDs light. Also, as MSAS-based filtering decreases or eliminate the blue peak, using it to filter light from LED will also reduce negative impacts of LEDs on starry skies observation, road safety and on vegetation.
Multilayer optical interference filters are well known in the prior-art. This type of filters has been used to filter light from various light sources. Several patents related to multi-layer layers are known to the inventors, including the following U.S. Pat. No. 2,412,496 by Dimmick, U.S. Pat. No. 2,624,238 by Widhop and Dimmick, U.S. Pat. No. 3,853,386 by Ritter and Pulker, U.S. Pat. No. 4,099,840 by van der Wal et al., U.S. Pat. No. 4,373,782 by Thelen, U.S. Pat. No. 4,838,629 by Maruyama et al., U.S. Pat. No. 5,007,710 by Nakajima et al., U.S. Pat. No. 5,274,661 by von Gunten et al., U.S. Pat. No. 5,926,317 by Cushing, U.S. Pat. No. 7,227,691 by Kamikawa. Several of these patents describe multilayer optical interference filters which include a plurality of layers of different thicknesses which, by optical interference, filters or reflects the light from a light source.
Also, filtering of light can be used for photo biological effects, notably to achieve an operative impact on the inhibition of melatonin secretion on humans. Known to the inventors are U.S. Pat. No. 5,274,403 by Gott, U.S. Pat. No. 7,520,607 by Casper et al., and U.S. patent application US 2012/0008326 by Jou. The design of such multilayer optical interference filters in accordance with photo biological parameters is more uncommon. Known to the inventors is U.S. Pat. No. 5,083,858 by Girerd which provides for sunglasses for multi-layer filtering of light for photo biologic purposes.
Such inventions do not filter the light as a function to a specific multi-maxima spectral response of melatonin suppression. Moreover, multi-layer optical interference filters have not been used for filtering light from LEDs directly from the light source in order to reduce the aforesaid undesirable effects.
The present invention overcomes some of the drawbacks of the prior-art by using a multi-layer optical interference filter which provides for the transmission in accordance with MSAS. One of the characteristics of MSAS is the presence of a secondary maximum on the spectrum function curve. MSAS can be characterized as the sum of two asymmetrical lognormal profiles. Such function cannot be processed through the use of a simple cut-off filter.